Monday, December 20, 2010

Working Backwards: cove and ovolo

Here's our goal:
Two profiles. One is 60 degrees of a concave circle with a radius of 6/16ths and the other is 90 degrees of a convex circle and has a radius of 4/16ths. We will use a 6 round and 4 hollow.
Let's remember what we've learned: a concave portion will begin with the tips of two fillets for the round to follow. Every convex portion will begin with 1 chamfered rabbet for the hollow to follow. Looking at the above profile there is one of each. We know that we'll need three rabbets. I hope the location of the first two are somewhat apparent.
Now we need to find where the third goes. The arc is 60 degrees of a circle so we know that the bottom of the rabbet needs to be on the circumference of the profile. We also know that the tips of the rabbets need to be parallel to the angle of the profile. There's only one solution.
And that's our third rabbet
Finally we have to figure out the chamfer. Remember, it needs to be parallel to the profile and on the profile.
Do you want to make a reverse ogee with a fillet? Reduce the depth of the red rabbet until the convex meets the concave.
Do you want to get rid of the fillet on top? Get rid of the green rabbet.
Do you want to switch the profile to make an ovolo and cove? switch the rabbets. (this is weird looking)
Add a second fillet between the two? Increase the width of the purple rabbet. (Not so weird looking now.)

 Regular ogee? Reduce the depth of the purple and red rabbets in this image up two up.

I'll stop now.

Please remember, this is perfect world stuff. Accuracy increases consistency while decreasing steering and scraping. 


  1. PERFECT! Thanks Matt!

    I have profiles for moldings needed for several clock cases. Until now, I had only the vaguest of ideas about how to make them. I could buy a full set of H&Rs [wouldn't you like that?] and see which ones do the job. Or, I could learn to do this sort of layout and select the correct planes.

    Thanks for the lessons.

  2. There is just one thing that I'm not getting. For the 90 degree circle, how do you make sure you get no plane marks given that your hollow has only a 60 degree arc? I have a feeling your answer is going to be “very carefully”.

    Randy Best (The Great White North)